Method for photon activation positron annihilation analysis

ABSTRACT

A non-destructive testing method comprises providing a specimen having at least one positron emitter therein; determining a threshold energy for activating the positron emitter; and determining whether a half-life of the positron emitter is less than a selected half-life. If the half-life of the positron emitter is greater than or equal to the selected half-life, then activating the positron emitter by bombarding the specimen with photons having energies greater than the threshold energy and detecting gamma rays produced by annihilation of positrons in the specimen. If the half-life of the positron emitter is less then the selected half-life, then alternately activating the positron emitter by bombarding the specimen with photons having energies greater then the threshold energy and detecting gamma rays produced by positron annihilation within the specimen.

RELATED APPLICATION

This application is a Divisional of pending U.S. application Ser. No.09/932,531, filed on Aug. 17, 2001.

CONTRACTUAL ORIGIN OF THE INVENTION

This invention was made with United States Government support underContract No. DE-AC07-99ID13727 awarded by the United States Departmentof Energy. The United States Government has certain rights in theinvention.

FIELD OF THE INVENTION

This invention relates generally to non-destructive testing of materialsand more specifically to methods and apparatus for performingnon-destructive testing of materials using positron annihilation.

BACKGROUND OF THE INVENTION

Non-destructive testing is the name commonly used to identify any of awide variety of techniques that may be utilized to examine materials fordefects without requiring that the materials first be destroyed. Suchnon-destructive testing of materials is advantageous in that allmaterials or products may be tested for defects. That is, after testing,acceptable (e.g., substantially defect-free) materials may be placed inservice, while the defective materials may be re-worked or scrapped, asmay be required. Non-destructive testing techniques are alsoadvantageous in that materials already in service may be tested orexamined in-situ, thereby allowing for the early identification ofmaterials or components that may be subject to in-service failure. Theability to test or examine new or in-service materials has madenon-destructive testing techniques of extreme importance in safety orfailure sensitive technologies, such as, for example, in aviation andspace technologies, as well as in nuclear power generation systems.

One type of non-destructive testing technique, generally referred to aspositron annihilation, is particularly promising in that it istheoretically capable of detecting fatigue damage in metals at itsearliest stages. While many different positron annihilation techniquesexist, as will be described below, all involve the detection of positronannihilation events in order to ascertain certain information about thematerial or object being tested.

In one type of positron annihilation technique, positrons from aradioactive source (e.g., ²² Na, ⁶⁸ Ge, or ⁵⁸ Co) are directed towardsthe material to be tested. Upon reaching the material, the positrons arerapidly “thermalized.” That is, the positrons rapidly lose most of theirkinetic energy by collisions with ions and free electrons present at ornear the surface of the material. After being thermalized, the positronsthen annihilate with electrons in the material. During the diffusionprocess, the positrons are repelled by positively charged nuclei, andthus tend to migrate toward defects such as dislocations in the latticesites where the distance to positively charged nuclei is greater. Inprinciple, positrons may be trapped at any type of lattice defect havingan attractive electronic potential. Most such lattice defects aresocalled “open volume” defects and include, without limitation,vacancies, vacancy clusters, vacancy-impurity complexes, dislocations,grain boundaries, voids, and interfaces.

Complete annihilation of a positron and an electron occurs when bothparticles collide and their combined mass is converted into energy inthe form of two (and occasionally three) photons (e.g., gamma rays). Ifthe positron and the electron are both at rest at the time ofannihilation, the two gamma rays are emitted in exactly oppositedirections (e.g., 180° apart) in order to satisfy the requirements ofthe conservation of momentum. Each annihilation gamma ray has an energyof about 511 keV, the rest energies of an electron and a positron. Inpositron annihilation techniques nearly all the positrons are at rest inthe defect or lattice sites. However, the electrons are not. Therefore,the momentum of the electron tends to determine the momentum of theannihilating pairs and cause the direction of the gamma rays to deviatefrom 180°. In addition to the momentum constraints, the energies of thegamma rays resulting from the annihilation may deviate slightly from 511keV, depending on the momentum of the electron. Accordingly, innon-destructive testing techniques utilizing positron annihilation, thedetection of the energies and relative angles of the gamma rays producedby the annihilation event are used to derive certain informationrelating to defects and other characteristics of the material or objectbeing tested.

While positron annihilation techniques of the type described above havebeen successfully used in the laboratory to detect defects in specimenmaterials, the technique has not been successfully utilized in fieldsettings. For one thing, the positrons from the external positron sourcebarely penetrate the surface of the material being tested. Consequently,such external positron source techniques are limited to near surfacemeasurements and generally must be conducted under controlled laboratoryconditions.

Partly in an effort to solve the depth limitations of the foregoingpositron annihilation testing technique, another type of positronannihilation technique has been developed that replaces the externalpositron source with an external neutron source. Neutrons from theneutron source are directed toward the material being tested. Givensufficient energies, the neutrons will, in certain materials, result inthe formation of isotopes that produce positrons. Such isotopes arecommonly referred to as positron emitters. The positrons then migrate tolattice defect sites, ultimately annihilating with electrons to producegamma rays. The resulting gamma rays are thereafter detected in themanner already described in order to derive information relating to thestructure of the material being tested.

The foregoing type of positron annihilation system is often referred toas a “neutron activated positron annihilation system” since it utilizesneutrons to trigger or induce the production of positrons. Sinceneutrons penetrate more deeply into the material being tested than dopositrons alone (e.g., from an external positron source), such neutronactivated positron annihilation systems are generally capable ofdetecting flaws deep within the material rather than merely on thesurface. Unfortunately, however, only a relatively few elements, such ascertain isotopes of copper, cobalt, and zinc, produce positrons inresponse to the neutron bombardment that are suitable for detectingflaws within the material. Consequently, neutron activated systems arelimited to use with materials that contain such responsive elements.

SUMMARY OF THE INVENTION

Non-destructive testing apparatus according to one embodiment of theinvention comprises a photon source. The photon source produces photonshaving predetermined energies and directs the photons toward a specimenbeing tested. The photons from the photon source result in the creationof positrons within the specimen being tested. A detector positionedadjacent the specimen being tested detects gamma rays produced byannihilation of positrons with electrons which are indicative of amaterial characteristic of the specimen being tested.

Also disclosed is a method that comprises the steps of: Providing aspecimen having at least one positron emitter therein; determining athreshold energy for activating the positron emitter; and determiningwhether a half-life of the positron emitter is less than a selectedhalf-life. If the half-life of the positron emitter is greater than orequal to the selected half-life, then activating the positron emitter bybombarding the specimen with photons having energies greater than thethreshold energy and detecting gamma rays produced by annihilation ofpositrons in the specimen. If the half-life of the positron emitter isless then the selected half-life, then alternately activating thepositron emitter by bombarding the specimen with photons having energiesgreater then the threshold energy and detecting gamma rays produced bypositron annihilation within the specimen.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative and presently preferred embodiments of the invention areshown in the accompanying drawing in which:

FIG. 1 is a schematic representation of one embodiment of thenon-destructive testing apparatus according to the present invention;

FIG. 2 is a flow chart representation of the method for non-destructivetesting according to one embodiment of the present invention;

FIG. 3 is a flow chart representation of the normal activation/analysisprocess;

FIG. 4 is a flow chart representation of the rapid activation/analysisprocess;

FIG. 5 is a schematic representation of one embodiment of apparatus forperforming the rapid activation/analysis process; and

FIG. 6 is a schematic representation of the various algorithms that maybe accessed by the data processing system.

DETAILED DESCRIPTION OF THE INVENTION

Non-destructive testing apparatus 10 according to one embodiment of thepresent invention is illustrated in FIG. 1 and may comprise a photonsource 12 and a detector 14. The photon source 12 produces photons(illustrated schematically by arrow 16) and directs the photons 16toward a material or specimen 18 being tested. The photons 16 from thephoton source 12 activate positron emitters (not shown) within thematerial or specimen 18, resulting in the creation of positrons (alsonot shown). Many of the positrons so formed ultimately annihilate withelectrons (not shown) within the specimen 18, resulting in the formationof gamma rays (illustrated schematically by arrow 20). The gamma rays 20resulting from the positron annihilations occurring within the specimen18 are detected by the detector 14 which produces raw data 22 related tothe detected gamma rays 20. A data collection and processing system 24operatively associated with the detector 14 is responsive to the rawdata 22 produced by the detector 14 and processes the raw data 22 toproduce output data 26 that are indicative of at least one materialcharacteristic of the specimen 18 being tested. Thereafter, the outputdata 26 may be presented in human-readable form an a suitable displaysystem 28.

As will be described in greater detail below, the method and apparatusof the present invention are suitable for use with materials orspecimens 18 that will produce positrons in response to photonbombardment from the photon source 12. One way for producing positronsinvolves the decay of neutron-deficient isotopes. In the presentinvention, the photons 16 from the photon source 12 produce suchneutron-deficient isotopes within the specimen 18 by removing or“knockingoff” neutrons from atoms within the specimen 18. Theneutron-deficient isotopes (referred to herein in the alternative as“positron emitters”) then decay into nonneutron-deficient atoms by theemission of positrons and neutrinos. Consequently, the bombardment of amaterial or specimen 18 containing certain isotopes amenable to the lossof neutrons by such photon bombardment will result in the formation ofpositrons within the material or specimen 18. This process is referredto herein as “photoneutron activation” or, simply, “photon activation.”Any material containing isotopes susceptible to such photon activationis suitable for use with the present invention.

A method 30 illustrated in FIG. 2 may be used to determine at least onematerial characteristic of the object or specimen 18. The first step 32in the method 30 involves determining whether the material or specimen18 to be analyzed includes one or more isotopes or “positron emitters”that are capable of photon activation. Stated another way, step 32 isused to identify those isotopes contained in the specimen 18 that willproduce positrons in response to photon bombardment. A next step 34 inthe method 30 determines the photon energy required to activate at leastone of the isotopes or positron emitters identified in step 32. As willbe described in greater detail below, the method and apparatus of thepresent invention allow a user to select for activation certain ones ofthe isotopes or positron emitters comprising the specimen 18.Accordingly, certain isotopes within the specimen 18 may be activated,while leaving other isotopes un-activated. The ability to selectivelyactivate certain positron emitters will allow a user to determineseveral material characteristics of the specimen 18, including, forexample, the amount or quantity of the selected positron emitterspresent in the specimen 18 as well as the locations of such positronemitters. Such information may be useful in ascertaining a wide range ofmaterial characteristics of the specimen 18, as will be described ingreater detail below.

Step 36 of the method 30 assesses the half-life of the photon activatedisotope or positron emitter to be activated. If the half-life of thepositron emitter is greater than a certain time (e.g., typically a fewminutes or greater), then the method 30 utilizes a normalactivation/analysis process 38 to test the specimen 18. Alternatively,if the half-life of the positron emitter is less than the certain time(e.g., typically on the order of tens of seconds or less), the specimen18 is tested or analyzed in accordance with a rapid activation/analysisprocess 40.

The normal activation/analysis process 38 is best seen in FIG. 3. Afirst step 42 in the normal activation/analysis process 38 involvesactivating the positron emitter or emitters (i.e., the isotope orisotopes identified in step 32). In one preferred embodiment, thepositron emitter is activated by bombarding the specimen 18 with photons16 from the photon source 12. It is generally preferred that the photons16 from the photon source 12 have sufficient energies to activate theselected isotope or positron emitter. For example, and as will bedescribed in greater detail below, photons having energies in the rangeof about 8 million electron volts (MeV) to about 22 MeV will activatemost of the isotopes (i.e., positron emitters) likely to be found inmany common materials. See, for example, Tables I and II. Alternatively,of course, photons having energies either above or below this range maybe used, depending on the particular isotope and on the particularmaterial characteristics to be detected.

The photon-activated positron emitters result in the production ofpositrons within the specimen 18. Such positrons diffuse or migratethrough the material comprising specimen 18 and tend to be attracted tovoids or other lattice defects having favorable electronic potentials.Ultimately, a significant number of positrons will annihilate withelectrons, resulting in the formation of gamma rays 20. Such gamma rays20 are detected in step 44 by the detector 14, which produces raw data22. The raw data 22 are then analyzed in step 46 to produce output data26 that are indicative of at least one material characteristic of thespecimen 18. The raw output data 26 may be displayed in suitable form onthe display system 28. See FIG. 1.

If the half life of the isotope or positron emitter to be activated isless than the certain time (e.g., typically on the order of tens ofseconds or less), as determined in step 36 (FIG. 2), the method 30executes the rapid activation/analysis process 40. With reference now toFIG. 4, the rapid activation/analysis process 40 involves alternatephoton bombardment and subsequent gamma ray detection of the specimen18. More specifically, the specimen 18 is first exposed to the photons16 from the photon source 12 for a selected time (e.g., 10 minutes) atstep 48. That is, the positron emitter or emitters are activated. Then,gamma rays 20 resulting from the annihilation of positrons withelectrons are detected via detector 14 at step 50. If a sufficientnumber of gamma rays 20 have been detected, as determined in step 53,the method 30 proceeds to step 54 wherein the data are analyzed toproduce output data 26 (FIG. 1) that are indicative of at least onematerial characteristic of the specimen 18. The output data 26 may bedisplayed in suitable form on the display system 28. Alternatively, ifan adequate number of gamma rays 20 have not been detected, asdetermined in step 53, the method 30 returns to step 48 wherein thespecimen 18 is again exposed to photons 16 from the photon source 12 forthe selected time. That is, the positron emitters comprising thespecimen 18 are re-activated. The activation and detection steps 48 and50 are repeated until a sufficient number of gamma rays 20 have beendetected.

The alternate photon activation and detection steps 48 and 50,respectively, may be accomplished in a variety of ways. For example, inone preferred embodiment, the specimen 18 is alternately moved betweenan activation position 56 and a detection position 58. See FIG. 5. Whilein the activation position 56, the specimen 18 is positioned adjacentthe photon source 12 so that the specimen receives photons 16 therefrom.Then, after having been exposed to the photons 16 for the selected time,the specimen 18 is moved to the detection position 58. While in thedetection position 58, the detector 14 detects gamma rays 20 emittedfrom the specimen 18 as a result of positron/electron annihilations.However, other arrangements are possible for accomplishing theactivation and detection steps 48 and 50. For example, in an alternativearrangement, the photon source 12 is alternately energized for theselected time period, then de-energized for a detection time period inwhich gamma rays 20 emitted from the specimen 18 are detected by thedetector 14.

Referring now to FIGS. 1 and 6, the data collection and processingsystem 24 may be provided with a data processing system 60 whichprocesses the raw data 22 from the detector 14 in accordance with one ormore algorithms in order to produce the output data 26 which areindicative of at least one material characteristic of the specimen 18.For example, in one preferred embodiment, the data processing system 60may process the data 22 in accordance with a Doppler broadeningalgorithm 62, a positron lifetime algorithm 64, and a three-dimensional(3-D) imaging algorithm 66.

The various algorithms (e.g., 62, 64, and 66) process the data 22 fromthe detector 14 in order to produce output data 26 which are indicativeof at least one material characteristic of the specimen 18. For example,the Doppler broadening algorithm 62 is useful in assessing thecharacteristics of lattice defects contained in the specimen 18, suchas, for example, damage resulting from mechanical and thermal fatigue,embrittlement, annealing, or manufacturing defects. The positronlifetime algorithm 64 is also useful in assessing the characteristics oflattice defects. In addition, information obtained from the meanlifetime of various defect components may be used to derive informationrelating to changing characteristics of the defects present in thespecimen 18. The 3-D imaging algorithm 66 may be used to in conjunctionwith either the Doppler broadening algorithm 62 or the positron lifetimealgorithm 64 to produce three-dimensional information regardinglocations of the lattice defects contained within the specimen 18.Alternatively, the raw gamma ray data 22 from the detector 14 may beprocessed in accordance with other algorithms that are now known in theart or that may be developed in the future to derive other types ofinformation, as would be obvious to persons having ordinary skill in theart after having become familiar with the teachings of the presentinvention. Consequently, the present invention should not be regarded aslimited to the particular processing algorithms shown and describedherein.

Regardless of the particular algorithm (e.g., 62, 64, or 66) that isused to process the raw data 22, the resulting output data 26 may bepresented in human-readable form on a suitable display system 28, suchas a CRT or LCD display. Alternatively, other types of display systemsmay be used to present the output data 26 in useable form.

For each algorithm, e.g., 62, 64, and 66, the data processing system 60may utilize a selective activation algorithm 68 in which certainisotopes or positron emitters in the specimen 18 are selected to beactivated. Stated simply, the selective activation algorithm 68 allowsthe data processing system 60 to set the energy level of the photons 16produced by the photon source 12. See FIG. 1. As mentioned above, theselective activation algorithm 68 provides the option to allow the userto activate certain of the isotopes or positron emitters comprising thespecimen 18.

A significant advantage of the present invention is that since thepositrons are produced within the material or specimen itself, ratherthan externally, the method and apparatus of the present invention maybe used to determine material characteristics of the specimen 18throughout the thickness (i.e., depth) of the specimen 18. Anothersignificant advantage of the present invention is that it may be used inconjunction with a wide range of materials, including metals, polymers,and composite materials, that were not heretofore available for fulldepth study by positron annihilation methods.

Still yet other advantages are associated with the ability to producethe positrons within the material specimen itself. For example, theinvention realizes increased sensitivity over conventional positronannihilation methods utilizing external positron sources in that thereis no extraneous background “noise” caused by annihilations external tothe specimen being analyzed. The increased sensitivity also allows othertypes of detectors (e.g., CdZnTe) to be used. Moreover, the surface ofthe specimen need not be specially prepared as is typically requiredwith systems involving external positron sources. The analysistechniques herein are also primarily dependent on and sensitive to theatomic characteristics of the specimen 18 and are not dependent on thephysical geometry of the specimen.

Another significant advantage of the present invention is that it may bemade specific to particular isotopes within the specimen. That is, byadjusting the energies of the photons 16 from the photon source 12, thephotons 16 may be used to selectively activate one or more positronemitters within the specimen 18 while leaving other positron emittersunactivated. Moreover, compared with conventional positron annihilationanalysis devices, the present invention may be made quite small andportable, thereby allowing the present invention to be readily andeasily utilized in field settings to analyze materials and specimensin-situ. The present invention may also be used to monitor materialsduring production and/or processing, thereby allowing for the earlydetection of non-compliant materials and for the possibility ofadjusting production parameters and processes to minimize the creationof non-compliant materials.

With the foregoing considerations in mind, non-destructive testingapparatus 10 according to one embodiment of the present invention isbest seen in FIG. 1 and may comprise a photon source 12 and a detector14. The photon source 12 produces photons 16 and directs the photons 16toward the specimen 18 being tested. It is generally preferred, but notrequired, that the photon source 12 be capable of producing photons 16having user adjustable (i.e. selectable) energies. The ability to adjustor select the energy of the photons 16 allows a user, in certainsituations, to selectively activate only certain ones of positronemitters or isotopes (not shown) comprising specimen 18 while leavingcertain other positron emitters un-activated. Alternatively, if suchselective activation of the positron emitters is not required or desiredin a particular application, the photon source 12 need not be providedwith capability to adjust the photon energy.

In one preferred embodiment having the ability to select the energies ofthe photons 16, the photon source 12 may comprise an electronaccelerator 70 for producing a stream of accelerated electrons, shownschematically in FIG. 1 as broken line 72. In order to produce thephotons 16 used to bombard the specimen 18, the accelerated electrons 72are directed toward a target 74 which produces the photons 16 inresponse to bombardment by the accelerated electron stream 72. Photonsgenerated in this manner are often referred to in the art asbremsstrahlung photons. There is a correlation between the energies ofthe electrons comprising the electron stream 72 and the photons producedby the target 74 in response to the electron bombardment. Consequently,photons 16 having specified energies can be produced by selecting oradjusting the energies of the electrons contained in the electron stream72. In the embodiment shown and described herein, the photons 16produced by the photon source 12 may be selected to have energies in therange of about 8 million electron volts (MeV) to about 22 MeV. Photons16 having energies in this range are often referred to as gamma rays.

In accordance with the foregoing considerations, then, the electronaccelerator 70 may comprise a linear accelerator of the type that arenow known in the art or that may be developed in the future that wouldbe suitable for the production of electrons in any of a wide range ofenergies. By way of example, in one preferred embodiment, the electronaccelerator 70 comprises a model 6000 linear accelerator available fromVarian Corp. of Palo Alto, Calif. Alternatively, equivalent devices fromthe same or other manufacturers may also be used. The target 74 whichproduces the photons 16 may comprise tungsten, although other materialsmay also be used. Of course, the photon source 12 and/or the variouscomponents comprising the photon source 12 (e.g., the electronaccelerator 70 and target 74) may be provided with suitable shieldingmaterials (not shown), to prevent the unwanted escape of radiation fromthe photon source 12.

In another embodiment of the invention, the photon source 12 maycomprise a radioactive isotope (not shown) suitable for producing gammaradiation having sufficient energies to activate at least one positronemitter contained in the specimen 18 to be tested. While the use of suchan isotopic gamma ray source has the advantage of dispensing with theneed for an electron accelerator and target, most isotopic gamma raysources do not readily lend themselves to producing gamma rays havingenergies that can be selected and varied by the user. However, the gammarays produced by certain isotopic sources do have known and generallypredictable energies, thus would be suitable for activating positronemitters having threshold (i.e., activation) energies generally at orbelow the energies of the gamma rays produced by the isotopic gamma raysource.

The detector apparatus 14 may be positioned adjacent the photon source12 and the specimen 18 so that the detector 14 receives gamma rays 20resulting from positron/electron annihilation events occurring withinthe specimen 18. Depending on the geometry of the particularinstallation, a shield 76 may be positioned between the photon source 12and the detector 14 to prevent gamma radiation from the photon source 12from being detected by detector 14. The detector 14 may be provided witha collimator 78 to collimate the gamma rays 20.

The detector 14 may comprise any of a wide range of gamma ray detectorsthat are now known in the art or that may be developed in the futurethat are or would be suitable for detecting gamma rays 20 produced bythe annihilation of positrons and electrons within the specimen 18.Accordingly, the present invention should not be regarded as limited toany particular type of gamma ray detector. However, by way of example,in one preferred embodiment, the detector 14 may comprise germaniumdetector of the type that is well-known in the art and readilycommercially available. Alternatively, the detector 14 could comprise acadmium-zinc-tellurium (CdZnTe) detector of the type that is alsowell-known in the art and readily commercially available. The collimator78 may comprise a variable slit type or other collimator.

It should also be noted that the present invention is not to be regardedas limited to use with only a single detector. Indeed, many of thealgorithms utilized by the present invention require, or at leastprefer, the use of more than one detector. For example, the positronlifetime algorithm 64 will generally require the use of at least twodetectors, one to detect the gamma rays 20 resulting from theannihilation events and one to detect “precursor” radiation associatedwith the production of the positrons themselves. Similarly, the 3-Dimaging algorithm 66 will also generally utilize at least two, andpreferably several, gamma ray detectors 14 in order to determine theposition of the positron/electron annihilation event within the specimen18. However, since the positron lifetime techniques and 3-D imagingtechniques are well-known in the art, as are the requirements for theparticular types and positions of detectors associated with suchtechniques, and since such multiple detectors could be easily providedby persons having ordinary skill in the art after having become familiarwith the teachings of the present invention, the particularconfigurations of such multiple detector systems as they could beutilized in the present invention will not be described in furtherdetail herein.

The data acquisition and processing system 24 is operatively associatedwith the detector apparatus 14 and receives raw data 22 from thedetector apparatus 14. In the embodiment shown and described herein, thedata acquisition and processing system 24 may comprise a dataacquisition system 80, as well as the data processing system 60. Thedata acquisition system acquires the raw data 22 from the detector andconverts it into a form suitable for use by the data processing system60. For example, in the case where the data processing system 60comprises a digital computer system, the data acquisition system 80 mayinclude an analog-to-digital (A/D) converter (not shown) suitable forconverting the analog data 22 from the detector 14 into digital datasuitable for use by the data processing system 60. Of course, otherarrangements and configurations are possible, as would be obvious topersons having ordinary skill in the art after having become familiarwith the teachings of the present invention. Consequently, the presentinvention should not be regarded as limited to any particular type ofdata acquisition system 80. However, by way of example, in one preferredembodiment, the data acquisition system 80 comprises a digital dataacquisition system available from EG&G of Oak Ridge, Tenn. as model no.“DSPEC+”. Alternatively, similar systems from the same or othermanufacturers may also be used. Initial analysis may be performed usingthe “Gamma Vision” software package commercially available from EG&G orthe “Genie 2000” software package commercially available from Canberraof Meriden, Conn. As will be described in greater detail below, in-depthanalysis is performed using algorithms to assess peak shapecharacteristics including shape comparisons, width ratios, and othershape characteristics.

The data processing system 60 may comprise a general purposeprogrammable digital computer, such as the ubiquitous personal computer,configured to operate in the manner described herein. Alternatively, thedata processing system 60 may comprise an application specific computerthat is customized to operate in accordance with the teachings herein.Regardless of the particular type of system that is used, the dataprocessing system 60 receives data from the data acquisition system 80and processes it in order to produce output data 26. The output data 26may be presented in human-readable form on any of a wide range ofdevices or systems, such as the display system 28, in order to indicatefor the user at least one material characteristic of the specimen 18being analyzed. By way of example, in one preferred embodiment, thedisplay system 28 may comprise a color display system (such as a CRT orLCD display) that is operatively associated with the data processingsystem 60. Alternatively, other systems may be used, as would be obviousto persons having ordinary skill in the art.

It is generally preferred, but not required, that the data processingsystem 60 also be operatively associated with the photon source 12. Suchan arrangement allows the data processing system 60 to control thefunction and operation of the photon source 12, such as, for example, toselect the desired photon energy, as well as to activate and deactivatethe photon source 12, as may be required by the rapidactivation/analysis process 40 (FIGS. 2 and 4) that may be utilized bythe method 30 of the present invention. Alternatively, of course, suchsystems integration need not be provided. For example, the operation ofthe photon source 12 instead could be manually controlled by the user.

Before proceeding with the description, it should be noted that themethod and apparatus of the present invention may be used with materialsor specimens 18 that will produce positrons in response to photonbombardment from the photons 16 produced by photon source 12. That is,the specimen 18 should comprise at least one positron emitter that, when“activated,” results in the production of positrons within the specimen18. As mentioned above, one way for generating positrons is through theformation within the specimen 18 of neutron-deficient isotopes, i.e.,positron emitters. Such neutron-deficient isotopes generally decay viathe emission of positrons and neutrinos. A list of positron emitters,the threshold gamma ray energies required to form or “activate” thepositron emitters, as well as their half-lives are presented herein asTables I and II. Table I includes those isotopes having half-lives onthe order of minutes or longer, whereas Table II includes short-livedisotopes having half-lives on the order of tens of seconds or less. Itis generally preferred that such short-lived isotopes (i.e., theisotopes listed in Table II) be analyzed with the rapidactivation/analysis process 40 shown and described herein.

Tables I and II may be used to readily identify those isotopes that maybe converted into positron emitters by photon bombardment as well as toestimate the photon energy required to form the positron emitters.

TABLE I Positron Emitters Threshold Element Reaction Half-Life UnitsEnergy MeV Chromium ⁵⁰Cr → ⁴⁹Cr 42.3 Minutes 20.5 Iron ⁵⁴Fe → ⁵³Fe 8.51Minutes 14 Nickel ⁵⁸Ni → ⁵⁷Ni 35.6 Hours 12 Copper ⁶⁵Cu → ⁶⁴Cu 12.7Hours 8 Copper ⁶³Cu → ⁶²Cu 9.74 Minutes 11 Zinc ⁶⁴Zn → ⁶³Zn 38.5 Minutes20.45 Zirconium ⁹⁰Zr → ⁸⁹Zr 4.18 Minutes 12.3 Molybdenum ⁹²Mo → ⁹¹Mo1.08, Minutes 12.5 15.5 Tin ¹¹²Sn → ¹¹¹Sn 35 Minutes 12.5 Antimony ¹²¹Sb→ ¹²⁰Sb 15.9 Minutes 10 Titanium ⁴⁶Ti → ⁴⁵Ti 3.1 Hours 13 Carbon ¹²C →¹¹C 20.3 Minutes 19 Nitrogen ¹⁴N → ¹³N 9.97 Minutes 10.5 Oxygen ¹⁵O →¹⁴O 122.2 Seconds ND Fluorine ¹⁹F → ¹⁸F 1.83 Hours 20 Phosphorus ³¹P →³⁰P 2.5 Minutes 10.9 Chlorine ³⁵Ci → ³⁴Ci 32.2 Minutes ND Potassium ³⁹K→ ³⁸K 7.6 Minutes 12.5 Gallium ⁶⁹Ga → ⁶⁸Ga 1.13 Hours ND Selenium ⁷⁴Se →⁷³Se 40 Minutes 12 Bromine ⁷⁹Br → ⁷⁸Br 6.45 Minutes ND Ruthenium ⁹⁶Ru →⁹⁵Ru 1.64 Hours ND Palladium ¹⁰²Pd → ¹⁰¹Pd 8.4 Hours ND Silver ¹⁰⁷Ag →¹⁰⁶Ag 24 Minutes 9.0 Cadmium ¹⁰⁶Cd → ¹⁰⁵Cd 55.5 Minutes ND Indium ¹¹³In→ ¹¹²In 14.4 Minutes ND Xenon ¹²⁴Xe → ¹²³Xe 2 Hours ND Cerium ¹³⁶Ce →¹³⁵Ce 17.7 Minutes ND Praseodymium ¹⁴¹Pr → ¹⁴⁰Pr 40 Minutes 7 Neodymium¹⁴²Nd → ¹⁴¹Nd 1.04 Minutes 9.5 Samarium ¹⁴⁴Sm → ¹⁴³Sm 8.83 Minutes 12.5Europium ¹⁵¹Eu → ¹⁵⁰Eu 12.8 Hours ND Erbium ¹⁶⁴Er → ¹⁶³Er 1.25 Hours ND

TABLE II Short Half-Life Positron Emitters Threshold Element ReactionHalf-Life Units Energy Mev Neon ²⁰Ne → ¹⁹Ne 17.2 Seconds ND Magnesium²⁴Mg → ²³Mg 11.32 Seconds 16 Aluminum ²⁷Al → ²⁶Al 6.3 Seconds ND Silicon²⁸Si → ²⁷Si 4.14 Seconds ND Sulfur ³²S → ³¹S 2.56 Seconds 15 Argon ³⁶Ar→ ³⁵Ar 1.77 Seconds ND

With reference now to FIG. 2, the method 30 of the present invention maybe used to determine at least one material characteristic of thespecimen 18. The first step 32 in the method 30 comprises determiningwhether the material or specimen 18 to be analyzed includes one or moreisotopes or “positron emitters” that are capable of photon activation.That is, step 32 involves a determination of the positron emitter oremitters to be activated. Tables I and II may be used for this purpose.For example, if it is known that the specimen 18 contains ⁵⁰Cr, photons16 having sufficient energy may be used to produce or form ⁴⁹Cr, apositron emitter.

The next step 34 in the method 30 involves a determination of the photonenergy required to activate at least one of the isotopes or positronemitters identified in step 32. For example, ⁵⁰Cr has a threshold energyof 20.5 MeV. Therefore, photons 16 having energies greater than or equalto this value will interact with ⁵⁰Cr to produce the positron emitter⁴⁹Cr. Of course, photons 16 having energies sufficient to activatechromium-50 will also activate other positron emitters contained in thespecimen 18 having lower threshold energies.

Step 36 of the method 30 assesses the half-life of the selected photonactivated isotope(s) or positron emitter(s). In this regard it should benoted that if the half-life of the positron emitter is greater than acertain time (e.g., generally on the order of minutes or longer), thenit will be advantageous to utilize the normal activation/analysisprocess 38 to test the specimen 18. Alternatively, if the half-life ofthe positron emitter is less than the certain time (e.g., on the orderof tens of seconds or less), the specimen 18 may be tested or analyzedin accordance with the rapid activation/analysis process 40. In theexample discussed herein involving chromium, Table I indicates that thehalf-life of the positron emitter ⁴⁹Cr is about 42.3 minutes. Therefore,it will be preferable to utilize the normal activation/analysis process38 for this positron emitter.

The normal activation/analysis process 38 is best seen in FIG. 3. Thefirst step 42 in the normal activation/analysis process 38 involvesactivating the positron emitter (i.e., the isotope or isotopesidentified in step 32). In one preferred embodiment, the positronemitter is activated by bombarding the specimen 18 with photons 16 fromthe photon source 12 having energies sufficient to activate the selectedpositron emitter or emitters, as the case may be. As mentioned above,photons having energies in the range of about 8 MeV to about 22 MeV willactivate most of the isotopes (i.e., positron emitters) likely to befound in many common materials. See, for example, Tables I and II.Alternatively, of course, photons having energies either above or belowthis range may be used, depending on the particular isotope and on theparticular material characteristics to be detected. In the exampleinvolving chromium-49, the photons 16 produced by the photon source 12should have energies of at least 20.5 MeV.

The photon-activated positron emitters result in the production ofpositrons within the specimen 18. Such positrons diffuse or migratethrough the material comprising specimen 18 and tend to be attracted tovoids or other lattice defects having a favorable electronic potential.Ultimately, a significant number of the positrons produced by thepositron emitter or emitters will annihilate with electrons, resultingin the formation of gamma rays 20. Such gamma rays 20 are detected instep 44 by the detector 14, which produces raw data 22. The raw data 22are then analyzed in step 46 to produce output data 26 indicative of atleast one material characteristic of the specimen 18. The output data 26may be displayed in suitable form on the display system 28. See FIG. 1.

If the half life of the isotope or positron emitter to be activated isless than a few tens of seconds, as determined in step 36, the method 30executes the rapid activation/analysis process 40. With reference now toFIG. 4, the rapid activation/analysis process 40 involves alternatephoton bombardment and subsequent gamma ray detection of the specimen18. More specifically, the specimen 18 is first exposed to the photons16 from the photon source 12 for a selected time at step 48. Then, gammarays 20 resulting from the annihilation of positrons with electrons aredetected via detector 14 at step 50. If a sufficient number of gammarays 20 have been detected, as determined in step 53, the method 30proceeds to step 54 wherein the data are analyzed to produce output data26 (FIG. 1) that are indicative of at least one material characteristicof the specimen 18. The output data 26 may be displayed in suitable formon the display system 28. Alternatively, if an adequate number of gammarays 20 have not been detected, the method 30 returns to step 48 whereinthe specimen 18 is again exposed to photons 16 from the photon source 12for a selected time. This rapid activation/analysis process 40 isrepeated until a sufficient number of gamma rays 20 have been detected.

The alternate photon activation and detection steps 48 and 50,respectively, may be accomplished in a variety of ways. For example,with reference now to FIG. 5, the specimen 18 could be alternately movedbetween an activation position 56 and a detection position 58. Asuitable mechanical arrangement (not shown) may be provided to move thespecimen 18 between the activation position 56 and the detectionposition 58. Alternatively, of course, the specimen 18 could remainstationary while the photon source 12 and detector 14 are moved. Again,a suitable arrangement for so moving the photon source 12 and detector14 could be easily arrived at by persons having ordinary skill in theart after having become familiar with the teachings of the presentinvention.

Regardless of the particular arrangement for moving the specimen 18between the activation position 56 and the detection position 58 (or formoving the photon source 12 and detector 14), the specimen 18, while inthe activation position 56, is positioned adjacent the photon source 12so that the specimen 18 receives photons 16 therefrom. Then, afterhaving been exposed to the photons 16 for the selected time, thespecimen 18 is moved to the detection position 58. While in thedetection position 58, the detector 14 detects gamma rays 20 emittedfrom the specimen 18 as a result of positron/electron annihilations. Thetimes in which the specimen 18 is located in the activation position 56and in the detection position 58 will vary depending on the particularpositron emitter or emitters involved and on the particular materialcharacteristics to be studied. However, the time during which thespecimen 18 remains in the activation position 56 should be sufficientto activate a sufficient number of positron emitters so that the gammarays 20 resulting from positron/electron annihilations will bedetectable by the detector 14. Similarly, the specimen 18 should remainin the detection position 58 for a time sufficient to detect gamma rays20 resulting from annihilation events. Generally speaking, the time thatthe specimen 18 should remain in the detection position 58 should be atleast equal to one half-life of the activated positron emitter oremitters, although the time could be longer or shorter than thehalf-life. In consideration of these matters, then, the presentinvention should not be regarded as limited to any particular times foreach position.

As was briefly mentioned above, other arrangements are possible foralternately activating the positron emitters then detecting the gammarays 20 resulting from annihilation events. For example, in anotherarrangement, the photon source 12 is alternately energized for theactivation time period, then de-energized for a detection time period inwhich gamma rays 20 emitted from the specimen 18 are detected by thedetector 14. Again, the activation time period should be set so as toactivate a sufficient quantity of positron emitters, whereas thedetection time period should encompass at least one half-life of theactivated positron emitter or emitters.

The data collection and processing system 24 may be provided with a dataprocessing system 60 which may process the data 22 from the detector 14in accordance with one or more algorithms in order to produce the outputdata 26 which are indicative of at least one material characteristic ofthe specimen 18. For example, with reference now to FIG. 6, in onepreferred embodiment, the data processing system 60 may process the data22 in accordance with a Doppler broadening algorithm 62, a positronlifetime algorithm 64, and a three-dimensional (3-D) imaging algorithm66. The various algorithms (e.g., 62, 64, and 66) process the data 22from the detector 14 in order to produce output data 26 which areindicative of at least one material characteristic of the specimen 18.

The Doppler broadening algorithm 62 is useful in assessing thecharacteristics of lattice defects contained in the specimen 18. Suchlattice defects may include, without limitation, damage resulting frommechanical and thermal fatigue, embrittlement, annealing, andmanufacturing defects. Doppler broadening techniques involve anassessment of the degree of broadening of the 511 keV peak associatedwith the gamma rays 20 produced by the positron/electron annihilationevent. Basically, a broadening of the peak is indicative of the presenceof one or more lattice defects. Several different types of Dopplerbroadening techniques have been developed and are being used in thepositron annihilation art and could be easily implemented in the presentinvention by persons having ordinary skill in the art after havingbecome familiar with the teachings of the present invention.Accordingly, the present invention should not be regarded as limited toany particular type of Doppler broadening technique. However, by way ofexample, in one preferred embodiment of the invention, the Dopplerbroadening algorithm 62 may comprise the Doppler broadening algorithmdescribed in U.S. Pat. No. 6,178,218 B1, which is specificallyincorporated herein by reference for all that it discloses.

The positron lifetime algorithm 64 is also useful in assessing thecharacteristics of lattice defects. For example, the positron lifetimealgorithm 64 may be used to obtain information as to whether the latticedefects comprise monovacancies, dislocations, slip zones, or particulateinclusions. In addition, information obtained from the mean lifetime ofvarious defect components may be used to derive information relating tochanging characteristics of the defects present in the specimen. Thepositron lifetime algorithm 64 basically involves a determination of thetime between positron formation and positron annihilation. In order todo so, the positron lifetime algorithm detects some precursor eventassociated with the formation of the positron, as well as the gamma rays20 produced by the positron annihilation event. The time between thesetwo events is the positron lifetime. In accordance with the foregoingprocess, systems utilizing positron lifetime analysis techniques usuallyutilize two separate detectors, one for detecting the precursor eventand the other for detecting the annihilation event. The system will alsousually include constant fraction discriminators, a time amplitudeconverter, as well as, a multi-channel analyzer system. However, sincesystems for detecting positron lifetimes, as well as the algorithmsutilized thereby, are well-known in the art and could be easily providedby persons having ordinary skill in the art after having become familiarwith the details of the present invention, the positron lifetimealgorithm 64, as well as the other systems and detectors that may berequired or desired, will not be described in further detail herein.

The 3-D imaging algorithm 66 may be used to in conjunction with eitherthe Doppler broadening algorithm 62 or the positron lifetime algorithm64 to produce three-dimensional information regarding locations of thelattice defects contained within the specimen 18. That is, in additionto determining the presence and characteristics of lattice defects(e.g., which may be accomplished by either the Doppler broadeningalgorithm 62 or the positron lifetime algorithm 64), the 3-D imagingalgorithm 66 is also able to determine the position within the specimen18 of the lattice defects. Consequently, the 3-D imaging algorithm 66 iscapable of providing a wealth of information regarding the internalstructure of the specimen 18.

As mentioned above, the 3-D imaging algorithm 66 will benefit from theuse of two or more separate detectors (e.g., detectors 14) in order toaccurately define the locations of the positron annihilation events.However, three dimensional imaging techniques of the type that may beutilized in the present invention, as well as multiple detectorarrangements for the use of the same, are also well-known in the art andcould be readily provided by persons having ordinary skill in the artafter having become familiar with the teachings of the presentinvention. For example, any of the imaging techniques and detectorarrangements that are currently utilized in positron emission tomography(PET) may be readily adapted for use with the present invention.Therefore, the particular 3-D imaging algorithm 66 (and detectorarrangements) that may be utilized in one embodiment of the presentinvention will not be described in further detail herein.

For each analysis algorithm, e.g., 62, 64, and 66, described above thedata processing system 60 may utilize a selective activation algorithm68. The selective activation algorithm 68 allows certain isotopes orpositron emitters in the specimen 18 to be activated. The selectiveactivation algorithm 68 is responsive to input from the user regardingeither the particular positron emitter or emitters to be activated orthe desired photon energy. The selective activation algorithm 68 thencontrols or operates the photon source 12 as necessary to producephotons 16 having energy levels suitable for activating the selectedpositron emitter or emitters. The selective activation algorithm 68allows the user to activate certain of the isotopes or positron emitterscomprising the specimen 18.

It is contemplated that the inventive concepts herein described may bevariously otherwise embodied and it is intended that the appended claimsbe construed to include alternative embodiments of the invention exceptinsofar as limited by the prior art.

1. A method, comprising: determining whether a specimen to be testedincludes at least one positron emitter therein that will be activated inresponse to photon bombardment; selecting a positron emitter to beactivated; determining a threshold photon energy required to activatethe selected positron emitter; determining a half-life of the selectedpositron emitter; and when the half-life of the selected positronemitter is less than a selected half-life, then performing a rapidactivation/analysis process, said rapid activation/analysis processcomprising: activating for an activation time the selected positronemitter by bombarding the specimen with photons having energies at leastas great as the threshold photon energy; detecting for a detection timegamma rays produced by annihilation of positrons with electrons in thespecimen; and repeating said steps of activating for an activation timeand detecting for a detection time until detecting a sufficient numberof gamma rays to determine at least one material characteristic of saidspecimen; when the half-life of the selected positron emitter is greaterthan or equal to the selected half-life, then performing a normalactivation/analysis process, said normal activation/analysis processcomprising: activating the selected positron emitter by bombarding thespecimen with photons having energies at least as great as the thresholdphoton energy; and detecting gamma rays produced by annihilation ofpositrons with electrons in the specimen.
 2. The method of claim 1,further comprising determining a positron lifetime based on the detectedgamma rays.
 3. The method of claim 1, further comprising using a Dopplerbroadening algorithm to determine the at least one materialcharacteristic.
 4. The method of claim 1, further comprising using athree dimensional imaging algorithm to determine a position within thespecimen of a positron/electron annihilation event.
 5. A method,comprising: providing a specimen comprising at least one positronemitter; determining a threshold energy for activating the at least onepositron emitter; comparing a half-life of the at least one positronemitter with a selected half-life; when the half-life of the at leastone positron emitter is greater than or equal to the selected half-life:activating the at least one positron emitter by bombarding the specimenwith photons having energies greater than the threshold energy; anddetecting gamma rays produced by annihilation of positrons withelectrons within the specimen; or, when the half-life of the at leastone positron emitter is less than the selected half-life: activating foran activation time the at least one positron emitter by bombarding thespecimen with photons having energies greater than the threshold energy;detecting for a detection time gamma rays produced by annihilation ofpositrons with electrons within the specimen; and repeating said stepsof activating for an activation time and detecting for a detection timeuntil detecting a sufficient number of gamma rays to determine at leastone material characteristic of said specimen.
 6. The method of claim 1,wherein the selected half-life is on the order of tens of seconds. 7.The method of claim 1, wherein selected half-life is about 17 seconds.8. The method of claim 1, wherein the detection time is about equal tothe half-life of the selected positron emitter.
 9. The method of claim1, wherein the rapid activation/analysis process further comprisesalternately moving the specimen between an activation position and adetection position, the activation position being adjacent a photonsource, the detection position being adjacent a detector.
 10. The methodof claim 1, wherein the rapid activation/analysis process furthercomprises alternately moving a photon source adjacent the specimenduring the activation time and away from the specimen during thedetection time and alternately moving a detector adjacent the specimenduring the detection time and away from the specimen during theactivation time.
 11. The method of claim 1, wherein the rapidactivation/analysis process further comprises activating a photon sourceto bombard the specimen with photons during the activation time andde-activating the photon source during the detection time.
 12. Themethod of claim 5, further comprising determining a positron lifetimebased on the detected gamma rays.
 13. The method of claim 5, furthercomprising using a Doppler broadening algorithm to determine the atleast one material characteristic.
 14. The method of claim 5, furthercomprising using a three dimensional imaging algorithm to determine aposition within the specimen of a positron/electron annihilation event.15. The method of claim 5, wherein the selected half-life is on theorder of tens of seconds.
 16. The method of claim 5, wherein selectedhalf-life is about 17 seconds.
 17. The method of claim 5, wherein thedetection time is about equal to the half-life of the at least onepositron emitter.
 18. The method of claim 5, further comprisingalternately moving the specimen between an activation position and adetection position, the activation position being adjacent a photonsource, the detection position being adjacent a detector.
 19. The methodof claim 5, further comprising alternately moving a photon sourceadjacent the specimen during the activation time and away from thespecimen during the detection time.
 20. The method of claim 19, furthercomprising alternately moving a detector adjacent the specimen duringthe detection time and away from the specimen during the activationtime.
 21. The method of claim 5, further comprising activating a photonsource to bombard the specimen with photons during the activation timeand de-activating the photon source during the detection time.
 22. Amethod, comprising: providing a specimen comprising at least onepositron emitter; determining a threshold energy for activating the atleast one positron emitter; comparing a half-life of the at least onepositron emitter with a selected half-life; when the half-life of the atleast one positron emitter is less than the selected half-life:alternately activating the at least one positron emitter and detectinggamma rays produced by annihilation of positrons within the specimenuntil detecting a sufficient number of gamma rays to determine at leastone material characteristic of said specimen.
 23. The method of claim22, wherein said activating the at least one positron emitter comprisesactivating for an activation time the at least one positron emitter bybombarding the specimen with photons having energies greater than thethreshold energy.
 24. The method of claim 23, further comprisingdetermining a positron lifetime based on the detected gamma rays. 25.The method of claim 23, further comprising using a Doppler broadeningalgorithm to determine the at least one material characteristic.
 26. Themethod of claim 23, further comprising using a three dimensional imagingalgorithm to determine a position within the specimen of apositron/electron annihilation event.
 27. The method of claim 23,wherein the selected half-life is on the order of tens of seconds. 28.The method of claim 23, wherein selected half-life is about 17 seconds.29. The method of claim 23, wherein detecting gamma rays is performedfor a time that is about equal to the half-life of the at least onepositron emitter.
 30. The method of claim 23, further comprisingalternately moving the specimen between an activation position and adetection position, the activation position being adjacent a photonsource, the detection position being adjacent a detector.
 31. The methodof claim 23, further comprising alternately moving a photon sourceadjacent the specimen during the activation time and away from thespecimen during the step of detecting gamma rays.
 32. The method ofclaim 31, further comprising alternately moving a detector adjacent thespecimen during the step of detecting gamma rays and away from thespecimen during the activation time.
 33. The method of claim 23, furthercomprising activating a photon source to bombard the specimen withphotons during the activation time and de-activating the photon sourceduring the step of detecting gamma rays.